Thermoelectrics, Photovoltaics and Thermal Photovoltaics for Powering ICT Devices and Systems

The conversion of heat into electricity through the thermoelectric effect and light into electricity through photovoltaic solar cells both allow useful amounts of power for a range of ICT systems from a few milli‐Watts (mW) for autonomous sensors up to kilo‐Watts (kW) for complete ICT computing or entertainment systems. Photovoltaics at the large scale can also be used to produce MW power stations suitable for the sustainable powering of high‐performance computing (HPC) and dataservers for cloud computing. This chapter provides a background to the physics of operation of both types of sustainable energy sources along with the fundamental limits of both technologies. The present performance is presented along with promising research directions to allow for a comparison of the useful power along with the limits for deployment of each approach to power ICT devices and systems. Finally, the developing field of thermal photovoltaics is reviewed, where the overall thermodynamic conversion efficiency of turning light into electricity and useful heat can be increased through the addition of thermoelectrics or heat transfer modules to a photovoltaic cell

Thermoelectric properties on Ge/Si1−xGex superlattices

Thermoelectric generation has been found to be a potential field which can be exploited in a wide range of applications. Presently the highest performances at room temperature have been using telluride-based devices, but these tech- nologies are not compatible with MEMs and CMOS processing. In this work Silicon and Germanium 2D superlattices have been studied using micro fabri- cated devices, which have been designed specifically to complete the thermal and electrical characterization of the different structures. Suspended 6-contact Hall bars with integrated heaters, thermometers and ohmic contacts, have been micro-fabricated to test the in-plane thermoelectric properties of p-type superlattices. The impact of quantum well thickness on the two thermoelectric figures of merit, for two heterostructures with different Ge content has been studied. On the other hand, etch mesa structures have been presented to study the cross-plane thermoelectric properties of p and n-type superlattices. In these experiments are presented: the impact of doping level on the two figures of merit, the impact of quantum well width on the two figures of merit, and the more efficient reduction of the thermal conductivity by blocking phonons with different wavelengths. The n-type results showed the highest figures of merit values reported in the literature for Te-free materials, presenting power factors of 12 mW/K2 · m, which exceeded by a factor of 3 the highest values reported in the literature. The results showed, that Si and Ge superlattices could compete with the current materials used to commercialise thermoelectric modules. In addi- tion, these materials have the advantage of being compatible with MEMs and CMOS processing, so that they could be integrated as energy harvesters to create complete autonomous sensors

Thermoelectric cross-plane properties on p- and n-Ge/SixGe1-x superlattices

Silicon and germanium materials have demonstrated an increasing attraction for energy harvesting, due to their sustainability and integrability with complementary metal oxide semiconductor and micro-electro-mechanical-system technology. The thermoelectric efficiencies for these materials, however, are very poor at room temperature and so it is necessary to engineer them in order to compete with telluride based materials, which have demonstrated at room temperature the highest performances in literature [1]. Micro-fabricated devices consisting of mesa structures with integrated heaters, thermometers and Ohmic contacts were used to extract the cross-plane values of the Seebeck coefficient and the thermal conductivity from p- and n-Ge/SixGe1-x superlattices. A second device consisting in a modified circular transfer line method structure was used to extract the electrical conductivity of the materials. A range of p-Ge/Si0.5Ge0.5 superlattices with different doping levels was investigated in detail to determine the role of the doping density in dictating the thermoelectric properties. A second set of n-Ge/Si0.3Ge0.7 superlattices was fabricated to study the impact that quantum well thickness might have on the two thermoelectric figures of merit, and also to demonstrate a further reduction of the thermal conductivity by scattering phonons at different wavelengths. This technique has demonstrated to lower the thermal conductivity by a 25% by adding different barrier thicknesses per period

Comparative study of annealed and high temperature grown ITO and AZO films for solar energy applications

We present the optical and electrical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures, as well as their potential performance when used as low emissivity coatings in hybrid photovoltaic-thermal systems. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 μm) to obtain a consistent model for the permittivity of each of the films. The best performance is found using the properties of the ITO film grown at 250 °C, with a state of the art resistivity of 0.2 mΩ-cm and an optimized thickness of 75 nm which leads to an estimated 50% increase in the extracted power compared to a standard diffused silicon solar cell. The Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties

Thermal emissivity of silicon heterojunction solar cells

The aim of this work is to evaluate whether silicon heterojunction solar cells, lacking highly emissive, heavily doped silicon layers, could be better candidates for hybrid photovoltaic thermal collectors than standard aluminium-diffused back contact solar cells. To this end, the near and mid infrared emissivity of full silicon heterojunction solar cells, as well as of its constituent materials – crystalline silicon wafer, indium tin oxide, n-, i- and p-type amorphous silicon – have been assessed by means of ellipsometry and FTIR. The experimental results show that the thermal emissivity of these cells is actually as high as in the more traditional structures, ~80% at 8 μm. Detailed optical modelling combining raytracing and transfer matrix formalism shows that the emissivity in these cells originates in the transparent conductive oxide layers themselves, where the doping is not high enough to result in a reflection that exceeds the increased free carrier absorption. Further modelling suggests that it is possible to obtain lower emissivity solar cells, but that a careful optimization of the transparent conductive layer needs to be done to avoid hindering the photovoltaic performance

The use of silicon-germanium superlattices for thermoelectric devices and microfabricated generators

Low dimensional structures such as superlattices have the potential to improve the thermoelectric properties of materials by engineering the scattering of phonons to reduce the thermal conductivity and therefore improve the thermeoelectric performance. Here we demonstrate the reduction in thermal conductivity in Ge/SiGe superlattices using multiple barrier engineering to scatter acoustic phonons at the key wavelengths for thermal transport. The approach allows ZT to be increased in wide quantum well superlattices through the reduction of heterointerfaces which scatter both electrons and phonons

High Efficiency Planar Geometry Germanium-on-silicon Single-photon Avalanche Diode Detectors

This paper presents the performance of 26 μm and 50 μm diameter planar Ge-on-Si single-photon avalanche diode (SPAD) detectors. The addition of germanium in these detectors extends the spectral range into the short-wave infrared (SWIR) region, beyond the capability of already well-established Si SPAD devices. There are several advantages for extending the spectral range into the SWIR region including: reduced eye-safety laser threshold, greater attainable ranges, and increased depth resolution in range finding applications, in addition to the enhanced capability to image through obscurants such as fog and smoke. The time correlated single-photon counting (TCSPC) technique has been utilized to observe record low dark count rates, below 100 kHz at a temperature of 125 K for up to a 6.6 % excess bias, for the 26 μm diameter devices. Under identical experimental conditions, in terms of excess bias and temperature, the 50 μm diameter device consistently demonstrates dark count rates a factor of 4 times greater than 26 μm diameter devices, indicating that the dark count rate is proportional to the device volume. Single-photon detection efficiencies of up to ~ 29 % were measured at a wavelength of 1310 nm at 125 K. Noise equivalent powers (NEP) down to 9.8 × 10-17 WHz-1/2 and jitters < 160 ps are obtainable, both significantly lower than previous 100 μm diameter planar geometry devices, demonstrating the potential of these devices for highly sensitive and high-speed imaging in the SWIR

3D LIDAR imaging using Ge-on-Si single–photon avalanche diode detectors

We present a scanning light detection and ranging (LIDAR) system incorporating an individual Ge-on-Si single-photon avalanche diode (SPAD) detector for depth and intensity imaging in the short-wavelength infrared region. The time-correlated single-photon counting technique was used to determine the return photon time-of-flight for target depth information. In laboratory demonstrations, depth and intensity reconstructions were made of targets at short range, using advanced image processing algorithms tailored for the analysis of single–photon time-of-flight data. These laboratory measurements were used to predict the performance of the single-photon LIDAR system at longer ranges, providing estimations that sub-milliwatt average power levels would be required for kilometer range depth measurements

Afterpulsing in Ge-on-Si single-photon avalanche diodes

In this letter, we investigate afterpulsing in 26 and 100 μm diameter planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors, by use of the double detector gating method with a gate width of 50 ns. Ge-on-Si SPADs were found to exhibit a 1% afterpulsing probability at a delay time of 200 μs and temperature of 78 K, and 130 μs at a temperature of 150 K. These delay times were measured with an excess bias of 3.5% applied, which corresponded to a single-photon detection efficiency of 15% at 1.31 μm . We demonstrate that reducing the detector diameter can also be an effective way to restrict afterpulsing in this material system